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Topic: Building my own Newtonian for the first time, but need help. (Read 435 times)

I used to image with a C8 until I was a fool and placed it on an elevated surface.... you can guess what happened. Anyway, since i'm out an OTA, I decided to build a new one. I initially was looking online at OTAs but just didn't think anything was a good fit for what I wanted to do. I am serious about imaging and I wanted the best, so I bought a Zambuto 8" F7 quartz mirror. There's only one problem: I have never made an OTA or even built anything so precise in my life, so now i'm trying to figure out how to best go about assembling an OTA successfully to use the mirror to it's full potential.

I was hoping to get guidance on assembly and my overall plan. When I bought the mirror, the guy I purchased it from threw in nearly everything (I think?) needed to make a Newtonian. He had the supplies but just hadn't put it all together. Here's what I am getting in the mail:

Regarding the mirror and mirror cell, the gentleman said this:"I picked up a Meade 8" mirror cell with the intentions of modifying it for a 6-point cell. This mirror is 20mm thick, so modifying the cell will work perfect with it. The cell has some cork stuck to it. It came that way and I've not done anything to it."What do you all think? Do I have all the supplies needed to make a functional OTA as-is?

I was trying to picture the assembly process in my head as best I could:

Step 1: Using a Bridgeport milling machine to make the holes in the Hastings Tube for the mirror cell screws and spider secondary holder, as well as cut out the hole for the focuser. I had never heard of a Hastings tube before, but I thought I could get a rolled tube from Parallax Instruments if I needed. Any difference in quality? Problem: How do I know the ideal spacing to drill the holes? All these super-precise measurements have me wondering: Some mirrors from Zambuto have the FL in inches on the side. But, does this mean so many inches to the actual surface of the secondary mirror? Even if I placed the mirror surface exactly at the FL distance, how about the need to shift the secondary for collimation? Doesn't that potentially change the distance and mess it all up?

Step 2: After preparing the tube with the holes, I thought that I should spray-paint or flock the inside of the tube black?

Step 3: Do I then install the spider and center the secondary?

Step 4: Attach the focuser on the OTA?

Step 5: Assemble a mirror cell

Step 6: Put the mirror in the mirror cell and firmly attach it to the back of the OTA using screws to keep it in place?You can tell by all these question marks that I have very little idea of what i'm doing, lol

Just want to know if my overall plan or order is missing a bunch of stuff, or if I need to change the order.

It will definitely be a big undertaking for me. A Newtonian isn't too complex with parts, but I think what is daunting is making sure the measurements are absolutely precise and that I make -or purchase- a really good mirror cell.

Starting with the basics. You have an 8" f/7 mirror. Is the f.l. exactly 56". You will need to know that.

A 10" diameter tube from Hastings will be fine. The radius is 5", but how far will you want the focus to be outside the tube? 3"?, 4"?

Lets pic 4". So that means the secondary will be 9" inside the focus of the primary.

8"/56" = x/9" , x = 1.28"

That 1.3" secondary may be too small for a 9" placement. Lets put the secondary at 8", then the cone of light is about 1.14". It is a little better. But if you're going to use the scope for photography, you will want to have a larger secondary. And you will want to have the focus further out from the tube. So you may not want to use that 1.3" secondary.Get some more measurements like: where will the primary be placed?What the distancefrom the center of the spider support to the center of the secondary? How far does the focuser extend beyond the tube? Where do you want the focus to be?

Draw everything out on graph paper, and scale it. You should be able to get a very good idea of where to place things.

You won't need a Bridgeport to make holes in the tube. I have a simple procedure for marking where to drill all the holes, but not tonight it's getting late.

Starting with the basics. You have an 8" f/7 mirror. Is the f.l. exactly 56". You will need to know that.

A 10" diameter tube from Hastings will be fine. The radius is 5", but how far will you want the focus to be outside the tube? 3"?, 4"?

Lets pic 4". So that means the secondary will be 9" inside the focus of the primary.

8"/56" = x/9" , x = 1.28"

That 1.3" secondary may be too small for a 9" placement. Lets put the secondary at 8", then the cone of light is about 1.14". It is a little better. But if you're going to use the scope for photography, you will want to have a larger secondary. And you will want to have the focus further out from the tube. So you may not want to use that 1.3" secondary.Get some more measurements like: where will the primary be placed?What the distancefrom the center of the spider support to the center of the secondary? How far does the focuser extend beyond the tube? Where do you want the focus to be?

Draw everything out on graph paper, and scale it. You should be able to get a very good idea of where to place things.

You won't need a Bridgeport to make holes in the tube. I have a simple procedure for marking where to drill all the holes, but not tonight it's getting late.

Thanks Garyth, I appreciate the feedback. That would be great if you could tell me how to drill the holes, but later when it's not so late, haha

If you plan to use 2" eye pieces, I highly recommend a two speed focuser. I have two Moonlite single speed focusers. One is on my driveway scope and I only use 1-1/4" Plossls with that scope. It's great. The other is on my travel scope. I use 17mm and 22mm Naglers with that scope and there isn't enough friction to hold those eye pieces steady with the single speed focuser: the eye pieces move in-and-out of focus.

That would be great if you could tell me how to drill the holes, but later when it's not so late, haha

I put masking tap on the metal then locate the hole and center punch them.

or draw out the holes on a piece of paper with the arc length of the tubes dia. that way you can divide the spacing into 3 or 4 even gaps.

I use hand drills for all the tube work. focuser hole done with a bi-metal hole saw 2.5in dia.[/quote]Thanks Pinbout, I actually didn't think about a hole saw, but now I will purchase a bit to use with something like a drill press, perhaps. Just for maximum accuracy. I wonder how I would keep things sufficiently accurate or stable using a hand drill?

To add to my first main post, now I have a lot more questions and concerns....after doing more reading, I am finding out about problems with mirror shift or unsecured mirrors with the old, standard flat mirrors. Also hearing about issues with clamps holding the mirror. Now I hear about conical mirrors from Royce, and to be honest those sound like a lot less hassle. However, I still bought this mirror with a firm belief in my head that it was a good choice because it would not have a risk of deforming under gravity due to the thick base and being able to support evenly with a mirror cell.

My goal is to use this F7 mirror for DSO imaging. You might ask, why the long F/ratio? Well, one target that I really want to get a great image of is the Ring Nebula M57. My favorite amateur picture that I have seen of it was taken with a DK 12.5 and 2,500 or something FL. I put his specs into 12dstring and got the arcsecs per pixel. I can achieve the same results using my 1,400 FL mirror and a smaller pixel camera. So, that's why I chose the mirror. I have a 5x powermate from Televue, so of course I could just attach it to even a shorter FL scope and call it a day to get my FL. However, I also think there are negatives to having more glass between the entrance and the camera chip.

How do I get around mirror shift? Do I stand any chance of producing decent astrophotography images with this mirror?

I use a protractor and cut a piece of cardboard the same diameter of the tube. I make marks, 3 at 120 degrees, and 4 at 90 degrees.The marks need to be transferred to the tube, but you want to keep things in position and aligned. So I use sheets of paper to place around the tube. They are taped together, and their edges need to be kept in line.Once the marks are transferred to the paper, the paper can slide along the tube.If you've made a scale layout of where your components are to be placed, it is easy now to measure the distances on the tube you need, and the marks where all your holes will go. Use a centerpunch for the holes, and drill them with a undersized drill first. Drill out the correct hole size later when you figure out what sizes you need.Using a hole saw for the large hole for the eyepiece holder is the way to go.Here a drawing that shows a layout. You can see that there are a lot of "?"s that you will have to figure out.

I think that with an 8" mirror, you should be good with the Meade mirror cell.

I would also think that it would be harder to mount a conical mirror.

Is your mirror a conical mirror, or a full 20mm thickness. I wouldn't worry about gravity distorting the mirror. You could go with a 9-point flotation systems, but an evenly supported mirror on the Meade cell should be fine.An even layer of cork should be ok. Just don't pinch the mirror with any hold down clips.

A lot of stuff you might be reading is for the larger mirrors. (not to take away from their information)

(still catching up)

You shouldn't need a heater for the secondary.

Once the primary is held securely and gently in the cell, there should be no mirror shift.

When you go to drill any holes in the tube, have the tube resting on some rags so it doesn't get scuffed up. You should be able to use a drill with one hand, and place your hand on the tube to steady it. Drilling the holes in the thin aluminum is very easy.

I can use a hole saw in a drill to make a hole. It is very easy, but I am used to drilling stuff like that, and know what to expect. (not trying to scare you.) Drill a small pilot hole first.

I use a protractor and cut a piece of cardboard the same diameter of the tube. I make marks, 3 at 120 degrees, and 4 at 90 degrees.The marks need to be transferred to the tube, but you want to keep things in position and aligned. So I use sheets of paper to place around the tube. They are taped together, and their edges need to be kept in line.Once the marks are transferred to the paper, the paper can slide along the tube.If you've made a scale layout of where your components are to be placed, it is easy now to measure the distances on the tube you need, and the marks where all your holes will go. Use a centerpunch for the holes, and drill them with a undersized drill first. Drill out the correct hole size later when you figure out what sizes you need.Using a hole saw for the large hole for the eyepiece holder is the way to go.

Thanks so much for your post Garyth!I am using that basic design you drew to help figure out my dimensions and measurements.

I also am realizing a new potential problem that I did not forsee previously: One reason I wanted to build my own OTA was that I wanted to use an aluminum tube to keep weight down to make it easy on my mount. However, after more reading, a guy mentioned that the Hastings tube is actually quite heavy, even for aluminum....I'm trying to figure out how much this will all weigh. My goal was to try to keep it under 20 lbs if possible. I'm not sure if I can do that. I would even consider truss-tube design if necessary, but I like the idea of a solid tube for maximizing contrast and minimizing mirror exposure to the elements.

". . .a guy mentioned that the Hastings tube is actually quite heavy, even for aluminum....I'm trying to figure out how much this will all weigh. My goal was to try to keep it under 20 lbs if possible."

for your 10" x 60" tube, with a wall thickness of .094", the tube weighs about 17.5 lbs.

". . .a guy mentioned that the Hastings tube is actually quite heavy, even for aluminum....I'm trying to figure out how much this will all weigh. My goal was to try to keep it under 20 lbs if possible."

for your 10" x 60" tube, with a wall thickness of .094", the tube weighs about 17.5 lbs.

I'm not sure of the tube thickness. If it were me I would have done .064 thickness.

I'm wondering if this thing is going to weigh too much for my mount....the only other option would be a truss design, but I don't like the idea of the mirror being out in the open like that.

My EQ6R-Pro shouldn't have a weight load over 22 lbs for imaging purposes, so now I have some tough choices to make.